Direct current bus control system

ABSTRACT

A direct current bus control system including a direct current bus connecting between an input power supply and a load, including a main stabilizing device including a first charge-&amp;-discharge element and a first power converter, and at least one sub-stabilizing device including a second charge-&amp;-discharge element, a charge element, or a discharge element, and including a second power converter, wherein the first power converter is configured to derive a bus voltage target value according to a power storage amount index of the first charge-&amp;-discharge element, and to bidirectionally pass direct current power, so that the voltage of the direct current bus matches the bus voltage target value, and the second power converter is configured to derive a current target value, and to pass direct current power, so that a current equal to the current target value flows.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application filed under 35U.S.C. 111 (a) claiming benefit under 35 U.S.C. 120 and 365 (c) of PCTInternational Application No. PCT/JP2018/043064 filed on Nov. 21, 2018and designating the U.S., which claims priority to Japanese PatentApplication No. 2017-223808 filed on Nov. 21, 2017. The entire contentsof the foregoing applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to a direct current bus control system.

2. Description of the Related Art

In recent years, as an alternative power source for fossil energy andnuclear energy, a power source system using renewable energy such assunlight, wind, and wave power has been attracting attention, and someof these have already been put into practical use.

In this type of power supply system, the generated power greatly variesdepending on the weather, season, location, and the like. For thisreason, in order to maintain the voltage of the direct current busconnected with the power supply system within a predeterminedpermissible range, it is preferable that the power supply such as aphotovoltaic cell or a wind power generator is directly connected to thedirect current bus via a large-capacity power converter having a wideinput range. However, in this case, an increase in the capacity of thepower converter leads to an increase in the size, complexity, and costof the entire system.

Here, for example, techniques described in PTL 1 to 3 are known asconventional techniques for stabilizing the power supplied from thepower supply system to the direct current bus and the direct current busvoltage. However, since the power fluctuation of the renewable energypower system is large, it is difficult to efficiently control the powerfluctuation of the direct current bus caused by an output fluctuation ofthe renewable energy power system and a load fluctuation.

CITATION LIST Patent Literature

PTL 1: Japanese Laid-open Patent Publication No. 2017-5944 (paragraphs[0101] to [0107], FIG. 1)

PTL 2: Japanese Laid-open Patent Publication No. 2005-224009 (paragraphs[0009] to [0022], FIG. 1, FIG. 3)

PTL 3: U.S. Pat. No. 5,800,919 (paragraphs [0050] to [0052], FIG. 12)

SUMMARY OF THE INVENTION Technical Problem

In view of the above, it is desirable to provide a control system forefficiently controlling a power fluctuation of a direct current buscaused by fluctuations of an input power supply and a load.

Means for Solving the Problems

A direct current bus control system for controlling a power fluctuationof a direct current bus connecting between an input power supply and aload includes a main stabilizing device including a firstcharge-&-discharge element and a first power converter and at least onesub-stabilizing device including a second charge-&-discharge element, acharge element, or a discharge element, and including a second powerconverter, wherein the first power converter is configured to derive abus voltage target value according to a power storage amount index ofthe first charge-&-discharge element, and to bidirectionally pass directcurrent power between the first charge-&-discharge element and thedirect current bus, so that the voltage of the direct current busmatches the bus voltage target value, and the second power converter isconfigured to derive a current target value according to a differencebetween: a threshold value of charge or discharge of the secondcharge-&-discharge element, the charge element, or the dischargeelement; and the voltage of the direct current bus, and is configured topass direct current power between: the second charge-&-dischargeelement, the charge element, or the discharge element; and the directcurrent bus, so that a current equal to the current target value flowsto or from the second charge-&-discharge element, the charge element, orthe discharge element.

Advantageous Effects of Invention

According to at least one embodiment, it is possible to provide acontrol system for efficiently controlling a power fluctuation of adirect current bus caused by fluctuations of an input power supply and aload.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall configuration diagram illustrating a direct currentbus control system according to an embodiment;

FIG. 2 is a configuration diagram illustrating another example of asub-stabilizing device according to an embodiment;

FIG. 3 is a block diagram illustrating an example of configuration of apower converter in a photovoltaic power generation system;

FIG. 4 is a block diagram illustrating an example of configuration of apower converter in a main stabilizing device;

FIG. 5 is a block diagram illustrating an example of configuration of apower converter in a sub-stabilizing device;

FIG. 6 is a block diagram illustrating an example of configuration of apower converter in a sub-stabilizing device;

FIG. 7 is a conceptual diagram schematically illustrating a relationshipbetween: charge-&-discharge powers of a power storage device, an inputpower of a water electrolysis cell, an output power of a fuel cell, andthe like; and a bus voltage;

FIG. 8A is a drawing for explaining an operation of the main stabilizingdevice;

FIG. 8B is a drawing for explaining an operation of the main stabilizingdevice;

FIG. 9A is a drawing for explaining an operation of the sub-stabilizingdevice;

FIG. 9B is a drawing for explaining an operation of the sub-stabilizingdevice;

FIG. 10A is a drawing for explaining an operation of the sub-stabilizingdevice;

FIG. 10B is a drawing for explaining an operation of the sub-stabilizingdevice;

FIG. 11 is a configuration diagram illustrating a model of a directcurrent bus control system used for simulation;

FIG. 12 is a block diagram illustrating a main part of the mainstabilizing device used for simulation;

FIG. 13 is a block diagram illustrating a main part of thesub-stabilizing device used for simulation;

FIG. 14 is a block diagram illustrating a main part of thesub-stabilizing device used for simulation;

FIGS. 15 (a)-(d) are waveform diagrams of voltages and currents ofrespective units, showing a result of a simulation; and

FIGS. 16 (a)-(d) are waveform diagrams of voltages and currents ofrespective units showing the result of the simulation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the present invention will be describedwith reference to the drawings.

FIG. 1 is an overall configuration diagram illustrating a direct currentbus control system according to this embodiment. The direct current buscontrol system illustrated in FIG. 1 includes, as an input power supply,a photovoltaic power generation system 10 and a wind-power generationsystem 20, which are renewable energy power supply systems. These powergeneration systems 10 and 20 are connected in parallel, and the outputsides of the power generation systems 10 and 20 are connected to thedirect current bus 70. The photovoltaic power generation system 10includes a photovoltaic cell 11 and a power converter 12, and thewind-power generation system 20 includes a wind-power generator 21 and apower converter 22.

The input power supply is not particularly limited. In a case where theinput power supply is a renewable energy power supply system, the inputpower supply may be a power supply system that uses wave power orgeothermal energy other than those described above, or may be a powersupply system such as hydropower (small hydropower) power generation,tidal power generation, marine current power generation, ocean thermalenergy power generation, or the like. The input power supply may also bea combination of those power supply systems including those listedabove.

Further, the number of power supply systems connected in parallel witheach other is not particularly limited.

The direct current bus 70 is connected to the main stabilizing device 30and the sub-stabilizing devices 40, 50, and 60, and is also connected tothe load 90.

The main stabilizing device 30 sets a variable bus voltage target valuewithin a predetermined permissible range around a reference bus voltage(i.e., a reference voltage of the direct current bus 70), and controlscharging and discharging of the power storage device 31 by operating thepower converter 32 so that the output voltage at the side of the directcurrent bus 70 matches the bus voltage target value.

The sub-stabilizing device 40 calculates an input-&-output currenttarget value on the basis of a difference between a charge-&-dischargethreshold value and the voltage of the direct current bus, and controlscharging and discharging of the power storage device 41 by operating thepower converter 42 so that an input-&-output current matches theinput-&-output current target value.

Herein, the power storage devices 31 and 41 are, for example, a battery(secondary battery), an electric double layer capacitor, a capacitor, aflywheel, a redox flow battery, or the like. The power converter 32 and42 are, for example, an isolated DC-DC converter or a chopper, and canbidirectionally pass direct current power as indicated by arrows.

The sub-stabilizing device 50 causes the power converter 52 to performDC-DC conversion so that the input-&-output current matches theinput-&-output current target value calculated based on the differencebetween the charge threshold value and the voltage of the direct currentbus, thus supplying direct current power to the water electrolysis cell51 (a kind of charge operation) to electrolyze water to generatehydrogen gas and oxygen gas. When direct current power generated byelectrochemical reaction of a fuel cell 61 is supplied to the directcurrent bus 70 via the power converter 62 (a kind of dischargeoperation), the sub-stabilizing device 60 causes the power converter 62to perform DC-DC conversion so that the input-&-output current matchesthe input-&-output current target value calculated based on thedifference between the discharge threshold value and the voltage of thedirect current bus.

The configurations of the sub-stabilizing device 50 and thesub-stabilizing device 60 described above are merely examples. Examplesof substitutes for the water electrolysis cell 51 include means forelectrochemically producing C—H bonds (CH₄, C₂H₄, and the like) andalcohol by reducing carbon dioxide and means for producing ammonia byreducing nitrogen. Examples of substitutes for the fuel cell 61 includea fuel cell using alcohol and power generation means that rotatesturbines and the like by combusting chemical substances (hydrogen,substances having C—H bonds, alcohol, ammonia, or the like).

FIG. 2 is another example of configuration of the sub-stabilizingdevice. As illustrated in the drawing, the sub-stabilizing devices 50and 60 explained above may be a sub-stabilizing device 50A of anintegrated structure in which the hydrogen storage device 53 is shared.

In FIG. 1, the power storage devices 31 and 41 are capable of chargingand discharging direct current power. The water electrolysis cell 51(and the hydrogen storage device 53 of FIG. 2) can convert directcurrent power into gas and accumulate the generated gas, and the fuelcell 61 (and the same hydrogen storage device 53) can perform powergeneration operation for converting the gas into direct current power.The power storage devices 31 and 41 constitute a charge-&-dischargeelement. The water electrolysis cell 51 (and the hydrogen storage device53) constitute a charge element. The fuel cell 61 (and the hydrogenstorage device 53) constitute a discharge element.

In this manner, each of the stabilizing devices 30, 40, 50, and 60 canbe deemed as a power buffer for exchanging direct current power with thedirect current bus 70 according to the operation of the power converters32, 42, 52, and 62. The main stabilizing device 30 and thesub-stabilizing device 40 are power buffers having charge-&-dischargefunction. The sub-stabilizing device 50 is a power buffer having acharge function. The sub-stabilizing device 60 is a power buffer havinga discharge function.

Only one main stabilizing device 30 having a setting function forsetting the bus voltage target value may be provided. Conversely, anyrequired number of sub-stabilizing devices may be provided according tothe number of power supply systems connected in parallel and the powerdemanded by the load 90.

The monitoring-&-instruction device 80 collects state information (e.g.,a voltage, a current, a temperature, and the like) about each of thepower generation systems 10 and 20, the main stabilizing device 30, andthe sub-stabilizing devices 40, 50, and 60 to monitor the state and theoperation, and generates an operation instruction (start and stopinstructions and the like), a charge-&-discharge threshold valueinstruction, and the like, on the basis of these monitor results.Various monitor signals and instructions can be transmitted and receivedbetween the monitoring-&-instruction device 80 and each of theabove-described units by wire or wirelessly.

The load 90 may be a direct current load such as a direct currentelectric motor and the like, or a DC/AC converter converting directcurrent power into alternating current power and an alternating currentload therefor. Alternating current power system may be connected to thedirect current bus 70 via a DC/AC converter.

Subsequently, the configuration of each part in FIG. 1 will bedescribed. In the configuration of FIG. 1, the photovoltaic powergeneration system 10 and the wind-power generation system 20 areprovided as input power supplies.

The photovoltaic power generation system 10 and the wind-powergeneration system 20 have a common function in that both convert a powergenerated using renewable energy into direct current power with thepower converters 12 and 22 and supply the direct current power to thedirect current bus 70. Therefore, the photovoltaic power generationsystem 10 will be described below as an example.

FIG. 3 is a block diagram illustrating an example of configuration ofthe power converter 12 in the photovoltaic power generation system 10.This power converter 12 includes a DC-DC conversion unit 12A and acontrol circuit 12B.

The DC-DC conversion unit 12A converts a direct current output voltageof the photovoltaic cell 11 into a direct current voltage of apredetermined magnitude according to an operation of a semiconductorswitching device, and outputs the converted direct current voltage tothe direct current bus 70. For example, the DC-DC conversion unit 12A isconstituted by a boost chopper.

In the control circuit 12B controlling the DC-DC conversion unit 12A, avoltage detector 12 a and a current detector 12 b detect a voltage and acurrent, respectively, which are output from the photovoltaic cell 11,and these detection values are input into the MPPT control unit 12 c.The MPPT control unit 12 c searches a maximum output point of thephotovoltaic cell 11 based on the hill climbing method and the like tooutput the maximum output point to the voltage-&-current control unit 12d.

The voltage-&-current control unit 12 d generates a driving pulseaccording to PWM (pulse width modulation) control and the like, andsends the driving pulse to a driving circuit 12 e. The driving circuit12 e turns ON and OFF a semiconductor switching device of the DC-DCconversion unit 12A on the basis of the driving pulse.

The voltage of the direct current bus 70 is detected by a voltagedetector 12 f, and this bus voltage detection value and a bus voltagetarget value sent from the main stabilizing device 30 described laterare input into a comparison unit 12 g. The comparison unit 12 ggenerates a control signal corresponding to a deviation between the busvoltage detection value and the bus voltage target value and outputs thecontrol signal to the voltage-&-current control unit 12 d.

The voltage-&-current control unit 12 d calculates a driving pulse forcausing the bus voltage detection value to match the bus voltage targetvalue on the basis of the above control signal. For example, in a casewhere the bus voltage detection value exceeds the bus voltage targetvalue, the voltage-&-current control unit 12 d performs controloperation (including operation stop) so as to decrease the outputvoltage of the DC-DC conversion unit 12A.

FIG. 4 is a block diagram illustrating an example of configuration ofthe power converter 32 in the main stabilizing device 30. This powerconverter 32 includes a DC-DC conversion unit 32A and a control circuit32B.

The DC-DC conversion unit 32A has a function of controlling charging anddischarging of the power storage device 31 by bidirectionally passingdirect current power between the direct current bus 70 and the powerstorage device 31. The DC-DC conversion unit 32A is constituted by anisolated DC-DC converter, a chopper, and the like having a semiconductorswitching device. The power storage device 31 is provided with a sensor31 a for detecting a voltage, a current, and a temperature. Theconfiguration of the control circuit 32B is as follows.

The voltage detector 32 a detects the voltage of the direct current bus70, and the bus voltage target value calculation unit 32 b calculates abus voltage target value according to the power storage amount index ofthe power storage device 31. The method for calculating the bus voltagetarget value will be explained later.

An example of the power storage amount index is a state of charge (SOC)obtained by integrating charge-&-discharge currents of the power storagedevice 31 detected by the sensor 31 a.

A subtractor 32 c calculates a difference between the bus voltage targetvalue and the bus voltage detection value, and outputs this voltagedifference to the charge-&-discharge control unit 32 d.

The charge-&-discharge control unit 32 d receives the voltage, thecurrent, and the temperature of the power storage device 31, and alsoreceives the charge-&-discharge threshold value. In view of inputinformation, the charge-&-discharge control unit 32 d generates adriving pulse by performing PWM control and the like so that the busvoltage detection value matches the bus voltage target value. A drivingcircuit 32 e turns on and off the semiconductor switching device of theDC-DC conversion unit 32A according to the driving pulse. In thismanner, the DC-DC conversion unit 32A controls charging and dischargingof the power storage device 31 to cause the bus voltage detection valueto match the bus voltage target value.

It should be noted that the charge-&-discharge threshold value of thepower storage device 31 may be set by the control circuit 32B, or may bereceived as an instruction from the monitoring-&-instruction device 80of FIG. 1.

FIG. 5 is a block diagram illustrating an example of configuration ofthe power converter 42 in the sub-stabilizing device 40 of FIG. 1. Thispower converter 42 includes a DC-DC conversion unit 42A and a controlcircuit 42B. The power converter 42 has a function similar to the powerconverter 32 of FIG. 4 in that the power converter 42 bidirectionallypasses direct current power between the direct current bus 70 and thepower storage device 41. Like the power storage device 31, the powerstorage device 41 is provided with a sensor 41 a for detecting avoltage, a current, and a temperature. The control circuit 42B includesa voltage detector 42 a, a comparison unit 42 b, a subtractor 42 c, acharge-&-discharge control unit 42 d, and a driving circuit 42 e.

The power converter 42 illustrated in FIG. 5 is different from the powerconverter 32 of FIG. 4 in the following features. In the control circuit42B, the charge-&-discharge control unit 42 d calculates aninput-&-output current target value on the basis of a difference betweena charge-&-discharge threshold value and the bus voltage detectionvalue. Further, the charge-&-discharge control unit 42 d controlscharging and discharging of the power storage device 41 so that theinput-&-output current of the DC-DC conversion unit 42A matches theinput-&-output current target value. Herein, the charge-&-dischargethreshold value may be a threshold value (a charge threshold value and adischarge threshold value) of charging and discharging of the powerstorage device 41. The input-&-output current target value may bedetermined in accordance with a difference between this threshold valueand the voltage of the direct current bus 70.

The comparison unit 42 b provided in the control circuit 42B comparesthe charge-&-discharge threshold value of the power storage device 41with the bus voltage detection value, and controls operation of thecharge-&-discharge control unit 42 d by outputting a charge instructionor a discharge instruction in accordance with a relationship inmagnitude between the charge threshold value or the discharge thresholdvalue and the bus voltage detection value. It should be noted that thecharge-&-discharge threshold value may be set by the control circuit42B, or may be received as an instruction from themonitoring-&-instruction device 80.

FIG. 6 is a block diagram illustrating an example of configuration ofthe power converter 52 in the sub-stabilizing device 50. This powerconverter 52 includes a DC-DC conversion unit 52A and a control circuit52B.

The DC-DC conversion unit 52A has a function of converting directcurrent power of the direct current bus 70 to a predetermined level andsupplying the converted direct current power to the water electrolysiscell 51. The DC-DC conversion unit 52A is constituted by an isolatedDC-DC converter, a chopper, and the like having a semiconductorswitching device. The water electrolysis cell 51 performs an operationelectrolyzing water using the direct current power supplied from theDC-DC conversion unit 52A and storing the generated hydrogen gas in anexternal storage device (not illustrated). In other words, the waterelectrolysis cell 51 performs a kind of charge operation.

The control circuit 52B controlling the DC-DC conversion unit 52A isconfigured in a manner substantially similar to the control circuit 42Bof FIG. 5.

More specifically, in the control circuit 52B of FIG. 6, a voltagedetector 52 a detects a voltage of the direct current bus 70, and asubtractor 52 c calculates a difference between a charge threshold valueand the bus voltage detection value, and this voltage difference isinput to the charge control unit 52 d. The bus voltage detection valueand the charge threshold value are also input to a comparison unit 52 b.When the bus voltage detection value is more than the charge thresholdvalue, the comparison unit 52 b outputs a charge instruction to thecharge control unit 52 d. Herein, the charge threshold value correspondsto a voltage at which the water electrolysis cell 51 startselectrolysis. In other words, the charge threshold value is a thresholdvalue for charging of the water electrolysis cell 51.

The charge control unit 52 d calculates an input-&-output current targetvalue on the basis of the voltage difference received from thesubtractor 52 c, generates a driving pulse as a charge instruction sothat the input-&-output current of the DC-DC conversion unit 52A matchesthe input-&-output current target value, and outputs the generateddriving pulse to a driving circuit 52 e. The driving circuit 52 e turnson and off a semiconductor switching device in the DC-DC conversion unit52A in accordance with the driving pulse, thus supplying direct currentpower to the water electrolysis cell 51 and electrolyzing water.

While the DC-DC conversion unit 52A controls the direct current powersupplied to the water electrolysis cell 51 according to the aboveoperation, the DC-DC conversion unit 52A operates so as to cause theinput-&-output current to match the input-&-output current target value.

In the sub-stabilizing device 60 of FIG. 1, the power generationoperation performed by the power generation operation by the fuel cell61 may be deemed as a discharge operation, and accordingly, the waterelectrolysis cell 51, the charge threshold value, and the charge controlunit 52 d of the sub-stabilizing device 50 illustrated in FIG. 6 may beread as the fuel cell 61, a discharge threshold value, and a dischargecontrol unit, respectively. In this case, the discharge threshold valuecorresponds to a start voltage of power generation by the fuel cell 61.

When the bus voltage detection value falls below the discharge thresholdvalue, the sub-stabilizing device 60 outputs a driving pulsecorresponding to the discharge instruction to the discharge control unitto cause the DC-DC conversion unit to perform operation to supply thepower generated by the fuel cell 61 to the direct current bus 70 via theDC-DC conversion unit.

While the DC-DC conversion unit controls the power to be generated bythe fuel cell 61 according to the above operation, the DC-DC conversionunit operates so as to cause the input-&-output current to match theinput-&-output current target value.

The water electrolysis cell 51 and the fuel cell 61 are also providedwith sensors for detecting voltages, currents, and temperatures, andthese detection values are input to the charge control unit 52 d and thedischarge control unit. For the sake of convenience, the sensors are notillustrated.

The charge threshold value and the discharge threshold value may be setby a corresponding control circuit, or may be received as an instructionfrom the monitoring-&-instruction device 80.

The power converters 12, 32, 42, and 52 illustrated in FIG. 3 to FIG. 6,and in particular, the configuration and the operation of the controlcircuits 12B, 32B, 42B, and 52B, are merely examples, and are notintended to limit the technical scope of the present invention any way,and it is to be understood that a configuration different from the abovemay be adopted.

Subsequently, FIG. 7 is a conceptual diagram schematically illustratinga relationship between: charge-&-discharge powers of the power storagedevice 41 of the sub-stabilizing device 40, an input power of the waterelectrolysis cell 51 of the sub-stabilizing device 50, and an outputpower of the fuel cell 61 of the sub-stabilizing device 60; and thevoltage of the direct current bus 70. The width of each triangularsymbol in FIG. 7 indicates the magnitude of the power. The wider thewidth of the triangular symbol is, the larger the value of the power is.

FIG. 7 illustrates an example in which the input power supply is arenewable energy power supply system. The renewable energy power supplysystem is, for example, the photovoltaic power generation system 10and/or the wind-power generation system 20 of FIG. 1. Thecharge-&-discharge operation of each unit is controlled in accordancewith the voltage of the direct current bus 70 to which the generatedpower is supplied and the charge-&-discharge threshold values and thelike of the power storage device 41, the water electrolysis cell 51, andthe fuel cell 61.

For example, as illustrated in a case (a) for the power storage device41, the higher the bus voltage is relative to the charge threshold valueof the power storage device 41, the larger the charge power supplied tothe power storage device 41 becomes, and the lower the bus voltage isrelative to the discharge threshold value of the power storage device41, the larger the discharge power discharged from the power storagedevice 41 becomes. Likewise, the higher the bus voltage is relative tothe charge threshold value of the water electrolysis cell 51, the largerthe charge power supplied to the water electrolysis cell 51 becomes, andthe lower the bus voltage is relative to the discharge threshold valueof the fuel cell 61, the larger the discharge power generated by thefuel cell 61 becomes.

A case (b) for the power storage device 41 relates to a case where thecharge threshold value and the discharge threshold value are set tovalues lower than the case (a) according to a reference bus voltage. Acase (c) relates to a case where the charge threshold value and thedischarge threshold value are set to values higher than the case (a). Asimilar setting change operation of the threshold value can also beperformed for the charge threshold value of the water electrolysis cell51 and the discharge threshold value of the fuel cell 61.

In this manner, the direct current power exchanged between the directcurrent bus 70 and the sub-stabilizing devices 40, 50, and 60 can beindividually adjusted by controlling the charge-&-discharge operation bychanging the charge threshold values and the discharge threshold valuesof the power storage device 41, the water electrolysis cell 51, and thefuel cell 61. In other words, the operation for the power buffer can bemore finely controlled in each of the sub-stabilizing devices 40, 50,and 60.

As described above, the changes of the charge threshold values and thedischarge threshold values can be performed on the basis of aninstruction from the monitoring-&-instruction device 80 or by the powerconverters 42, 52, and 62.

FIGS. 8A and 8B are drawings for explaining an operation of the mainstabilizing device 30. As indicated by a thick broken line in FIG. 8A,the main stabilizing device 30 passes direct current power between thedirect current bus 70 and the power storage device 31, and controlscharging and discharging of the power storage device 31. The controlcircuit 32B in the power converter 32 sets a bus voltage target value onthe basis of the power storage amount index (for example, a state ofcharge) of the power storage device 31 according to characteristics asillustrated in FIG. 8B, for example.

Within a permissible range of the voltage of the direct current bus 70,this bus voltage target value is set to a higher value as the powerstorage amount index is larger, and is set to a lower value as the powerstorage amount index is smaller. The control circuit 32B controls theDC-DC conversion unit 32A so as to cause the bus voltage detection valueto match this bus voltage target value.

FIGS. 9A and 9B are drawings for explaining operations of thesub-stabilizing devices 40 and 50.

As indicated by thick broken lines in FIG. 9A, the power converter 42 ofthe sub-stabilizing device 40 charges the power storage device 41 byusing the direct current power of the direct current bus 70, and thepower converter 52 of the sub-stabilizing device 50 supplies the directcurrent power of the direct current bus 70 to the water electrolysiscell 51 to electrolyze water.

The charge characteristics in this case are as illustrated in FIG. 9B.Each of the power converters 42 and 52 is controlled so that, the higherthe voltage of the direct current bus 70 is relative to the chargethreshold value of the power storage device 41 or the water electrolysiscell 51, the larger the charge current becomes. FIGS. 10A and 10B aredrawings for explaining operations of the sub-stabilizing devices 40 and60.

As illustrated by thick broken lines in FIG. 10A, the power converter 42of the sub-stabilizing device 40 causes the power storage device 41 todischarge and supply direct current power to the direct current bus 70,and the power converter 62 of the sub-stabilizing device 60 causes thefuel cell 61 to perform power generation operation to supply directcurrent power to the direct current bus 70.

The charge characteristics in this case are as illustrated in FIG. 10B.Each of the power converters 42 and 62 is controlled so that, the lowerthe voltage of the direct current bus 70 is relative to the dischargethreshold value of the power storage device 41 or the fuel cell 61, thelarger the discharge current becomes.

Subsequently, a simulation performed to verify the effects of thedisclosed technology will be described.

FIG. 11 illustrates a model of a direct current bus control system usedfor simulation, and includes the photovoltaic power generation system10, the main stabilizing device 30, the sub-stabilizing devices 50 and60, the direct current bus 70, and the load 90.

Herein, the power converter 12 of the photovoltaic power generationsystem 10 is assumed to perform MPPT (maximum power point tracking)control for changing current drawn and voltage with every 0.1 [sec].

The power converter 32 of the main stabilizing device 30 measures acharge-&-discharge current of the power storage device 31 to derive anestimated power storage amount index with every 0.1 [sec]. The busvoltage target value is calculated on the basis of this estimated powerstorage amount index, a reference power storage amount index, and areference bus voltage.

FIG. 12 is a block diagram illustrating a main part of the mainstabilizing device 30. The configuration illustrated in FIG. 12 includesa pulse generator 1, an estimated power storage amount index calculationunit 2, subtractors 3 and 6, a gain multiplier 4, an adder 5, and a PID(proportional integral derivative) controller 7. K1 is a constantcorresponding to the reference power storage amount index, and K2 is aconstant corresponding to the reference bus voltage.

FIG. 13 is a block diagram illustrating a main part of thesub-stabilizing device 50 used for simulation. The configuration asillustrated in FIG. 13 includes: a subtractor 9 configured to derive adifference between the bus voltage detection value and the reference busvoltage; a PID controller 110 configured to operate so as to eliminatethe difference; and a memory 111, and calculates a current to be outputto the water electrolysis cell 51.

FIG. 14 is a block diagram illustrating a main part of thesub-stabilizing device 60 used for simulation. The configuration asillustrated in FIG. 14 includes: a subtractor 112 configured to derive adifference between the reference bus voltage and the voltage detectionvalue of the fuel cell 61; and a PID controller 113 configured tooperate so as to eliminate the difference, and calculates a powergeneration current by the fuel cell 61.

FIGS. 15 (a)-(d) and FIGS. 16 (a)-(d) are waveform diagrams of voltagesand currents of respective units, showing a result of a simulation.

FIG. 15 (a) indicates a voltage of the photovoltaic cell 11, FIG. 15 (b)indicates a current of the photovoltaic cell 11, FIG. 15 (c) indicates abus voltage, and FIG. 15 (d) indicates a load current. FIG. 16 (a)indicates a voltage of the power storage device 31, FIG. 16 (b)indicates a current of the power storage device 31, FIG. 16 (c)indicates a current of the water electrolysis cell 51, and FIG. 16 (d)indicates a current of the fuel cell 61 (including a leakage current ina normal state).

Herein, voltages and currents of the respective units were simulated, inwhich at a time t1, the output current of the photovoltaic cell 11 rose,and thereafter, at a time t2, the load 90 was activated, and at a timet3 (=40 [sec]), the current of the photovoltaic cell 11 decreased, andat a time t5 (=80 [sec]), the load current became zero.

In response to the start of power generation by the photovoltaic cell 11at time t1, when the bus voltage became higher than the charge thresholdvalue of the power storage device 31 and the water electrolysis cell 51,the power storage device 31 and the water electrolysis cell 51 startedto be charged, and accordingly, the voltage of the power storage device31 became higher (FIG. 16 (a)), and the input current of the waterelectrolysis cell 51 increased (FIG. 16 (c)).

As a result, the power storage amount index of the power storage device31 increased, and therefore, the bus voltage target value also becamehigher after time t1 (FIG. 15 (c)).

When the load 90 was activated at time t2, the bus voltage slightlydecreased (FIG. 15 (c)).

At time t3, the current and the bus voltage of the photovoltaic cell 11decreased, and when the bus voltage fell below the discharge thresholdvalue of the power storage device 31, the power storage device 31started to discharge, and accordingly, the voltage of the power storagedevice 31 decreased (FIG. 16 (a), FIG. 16(b)). In FIG. 16 (b), a currentin the negative direction is a discharge current. Likewise, after timet3, the current of the water electrolysis cell 51 also became zero.

When the bus voltage fell below the discharge threshold value of thefuel cell 61, the output current of the fuel cell 61 increased at a timet4 immediately after time t3 (FIG. 16 (d)), and after time t4, the busvoltage and the voltage of the power storage device 31 were maintainedat a substantially constant value (FIG. 15 (c), FIG. 16 (a)).

Thereafter, when, at time t5, the load current became zero (FIG. 15(d)), the bus voltage began to rise, and when the bus voltage exceededthe discharge threshold value of the fuel cell 61, the discharge currentfrom the fuel cell 61 became zero (FIG. 15 (c), FIG. 16 (d)). Since thebus voltage exceeded the charge threshold value of the power storagedevice 31, the power storage device 31 started to be charged, andcontinued to be charged until a time t6 (FIG. 16 (a), FIG. 16 (b)).

Furthermore, when the bus voltage exceeded the charge threshold value ofthe water electrolysis cell 51, the input current to the waterelectrolysis cell 51 increased until the voltage of the waterelectrolysis cell 51 became substantially equal to the bus voltage, andafter time t6, the input current became a substantially constant value(FIG. 16 (c)).

In the above operation, the bus voltage target value in FIG. 15 (c) wascalculated by the block diagram of the main stabilizing device 30 asillustrated in FIG. 12. The bus voltage detection value closely followedthe bus voltage target value.

In response to the change in the output of the photovoltaic cell 11 andthe change in the load current during the simulation period, the mainstabilizing device 30 having the power storage device 31, thesub-stabilizing device 50 having the water electrolysis cell 51, and thesub-stabilizing device 60 having the fuel cell 61 are operated as powerbuffers by autonomously performing charge operation or dischargeoperation in accordance with the relationship in magnitude between thecorresponding charge-&-discharge threshold value and the bus voltage. Asa result, it was found that the bus voltage detection value wasmaintained in a predetermined permissible range (in a range ofapproximately from 379.7 [V] to 380.5 [V]). When the supply power of thedirect current bus 70 is considered, as described above, the bus voltagedetection value closely followed the bus voltage target value, and wasmaintained substantially constantly within the predetermined permissiblerange. As a result, the power fluctuation of the direct current bus 70substantially matches the change in the current value. Therefore, in thedirect current bus control system according to the present invention,with the control of voltage performed by the main stabilizing device 30and the control of the current performed by the sub-stabilizing devices50 and 60, the power fluctuation of the direct current bus 70 can becontrolled.

In a case where multiple sub-stabilizing devices having charge functions(for example, sub-stabilizing devices having water electrolysis cells)are connected to the direct current bus 70, a quickly respondingsub-stabilizing device operates preferentially to absorb the powerfluctuation of the direct current bus 70, and the remainingsub-stabilizing devices may not operate. Such a situation is notdesirable from the viewpoint of equalizing the operations of thedevices. A problem similar to the above may occur in a case wheremultiple sub-stabilizing devices having discharge functions (forexample, sub-stabilizing devices having fuel cells) are connected to thedirect current bus 70 or in a case where multiple sub-stabilizingdevices having charge-&-discharge functions (for example,sub-stabilizing devices having power storage devices) are connected tothe direct current bus 70.

For the above problem, a Droop control for reducing the output voltageaccording to the increase of the output current may be applied tomultiple sub-stabilizing devices having the same function (a chargefunction or a discharge function) to adjust the Droop rates so that theload (e.g., a utilization ratio or operation responsibility) may bedistributed to the devices with a predetermined ratio.

Instead of the operation method for equalizing the operations ofmultiple sub-stabilizing devices as described above, in view of areaction responsiveness, a charge capacity, and the like of eachsub-stabilizing device, it may be considered to employ a method ofoperating with prioritization of charge powers and discharge powers,such as, for example, causing a certain sub-stabilizing device tooperate in a state close to a fully charged state and causing anothersub-stabilizing device to operate in a substantially completelydischarged state.

This application claims priority based on Japanese Patent ApplicationNo. 2017-223808 filed with the Japan Patent Office on Nov. 21, 2017, theentire content of which is incorporated herein by reference.

What is claimed is:
 1. A direct current bus control system forcontrolling a power fluctuation of a direct current bus connectingbetween an input power supply and a load, the direct current bus controlsystem comprising: a main stabilizing device including a firstcharge-&-discharge element and a first power converter; and at least onesub-stabilizing device including a second charge-&-discharge element, acharge element, or a discharge element, and including a second powerconverter, wherein the first power converter is configured to derive abus voltage target value according to a power storage amount index ofthe first charge-&-discharge element, and to bidirectionally pass directcurrent power between the first charge-&-discharge element and thedirect current bus, so that the voltage of the direct current busmatches the bus voltage target value, and the second power converter isconfigured to derive a current target value according to a differencebetween: a threshold value of charge or discharge of the secondcharge-&-discharge element, the charge element, or the dischargeelement; and the voltage of the direct current bus, and is configured topass direct current power between: the second charge-&-dischargeelement, the charge element, or the discharge element; and the directcurrent bus, so that a current equal to the current target value flowsto or from the second charge-&-discharge element, the charge element, orthe discharge element.
 2. The direct current bus control systemaccording to claim 1, wherein the at least one sub-stabilizing devicecomprises a plurality of sub-stabilizing devices, and the plurality ofsub-stabilizing devices includes at least one of a sub-stabilizingdevice having the charge element and a sub-stabilizing device having thedischarge element.
 3. The direct current bus control system according toclaim 1, wherein the first power converter is configured to determinethe bus voltage target value within a predetermined permissible range ofthe voltage of the direct current bus.
 4. The direct current bus controlsystem according to claim 1, wherein the first power converter includesa first DC-DC converter, and is configured to control the first DC-DCconverter based on a result obtained by comparing the voltage of thedirect current bus with a charge-&-discharge threshold value of thefirst charge-&-discharge element connected to the first power converter.5. The direct current bus control system according to claim 4, furthercomprising a monitoring-&-instruction device capable of sending andreceiving an operation instruction and state information about the mainstabilizing device and the sub-stabilizing device, wherein themonitoring-&-instruction device is configured to transmit thecharge-&-discharge threshold value to the first power converter.
 6. Thedirect current bus control system according to claim 1, wherein thesecond power converter includes a second DC-DC converter, and isconfigured to control the second DC-DC converter based on a resultobtained by comparing a charge threshold value of the secondcharge-&-discharge element or the charge element or a dischargethreshold value of the second charge-&-discharge element or thedischarge element with the voltage of the direct current bus.
 7. Thedirect current bus control system according to claim 6, furthercomprising a monitoring-&-instruction device capable of sending andreceiving an operation instruction and state information about the mainstabilizing device and the sub-stabilizing device, wherein themonitoring-&-instruction device is configured to transmit the chargethreshold value or the discharge threshold value to the second powerconverter.
 8. The direct current bus control system according to claim1, wherein as the power storage amount index increases, the bus voltagetarget value increases accordingly.
 9. The direct current bus controlsystem according to claim 1, wherein the power storage amount index is astate of charge of the first charge-&-discharge element.
 10. The directcurrent bus control system according to claim 1, wherein as a differencebetween the threshold value and the voltage of the direct current busincreases, the second power converter increases a charge-&-dischargecurrent of the second charge-&-discharge element, the charge element, orthe discharge element, accordingly.
 11. The direct current bus controlsystem according to claim 1, further comprising a renewable energy powersupply system as the input power supply.
 12. The direct current buscontrol system according to claim 11, further comprising the load.